Astronomy Chapter 12

Type 1 supernova

1 possible explosive death of a star. A white dwarf in a binary star system can accrete enough mass that it cannot support its own weight, the star collapses and temperatures becomes high enough for carbon fusion to occur. Fusion begins throughout the white dwarf simultaneously, and an explosion results.

Type 2 supernova

1 possible explosive death of star, in which the highly evolved stellar core rapidly implodes and then explodes, destroying the surrounding star.

horizontal branch

region of the HR diagram where post main sequence stars again reach hydrostatic equilibrium. At this point, the star is burning helium in its core and fusing hydrogen in a shell surrounding the core.

main sequence turn off

special point on the HR diagram for a cluster, indicative of the cluster's age. If all the stars in the cluster are plotted, the lower mass stars will trace out the main sequence up to the point where stars begin to evolve off the main sequence towards the red giant branch. The point where stars are just beginning to evolve off is this turnoff.

nova (novae)

a star that suddenly increases in brightness, often by a factor by as much as 10,000, then slowly fades back to its original luminosity. This is the result of an explosion on the surface of a white dwarf star caused by matter falling onto its surface from the atmosphere of a binary companion.

helium flash

an explosive event in the post main sequence evolution of a low mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves stable equilibrium again.

accretion disk

flat disk of matter spiraling down onto the surface of a neutron star or black hole. Often, the matter originated on the surface of a companion star in a binary star system.

planetary nebula

the ejected envelope of a red giant star, spread over a volume roughly the size of our solar system.

black dwarf

the end point of the evolution of an isolated low mass star. After the white dwarf stage, the star cools to the point that it is a dark, "clincker" in interstellar space.

core hydrogen burning

the energy burning stage for main sequence stars, in which the helium in produced by hydrogen fusion in the central region of the star. A typical star spends up to 90 percent of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by this process.

supernova remnants

the scattered glowing remains from a supernova that occurred in the past.

red giant branch

the section of the evolutionary track of a star corresponding to an intense hydrogen shell burning. Which drives a steady expansion and cooling of the outer envelope of the star. As the star gets larger in radius and its surface temperature cools, it becomes a red giant.

sub-giant branch

the section of the evolutionary track of a star that corresponds to changes that occur just after hydrogen is depleting in the core, and core hydrogen burning ceases. Shell hydrogen burning heats outer layers of the star, which causes a general expansion of the stellar envelope.